The present invention relates to a scroll compressor suitable for use in, for example, refrigerators and air-conditioners and, more particularly, to a hermetic scroll compressor having a control valve capable of ensuring a stable supply of lubricating oil to the bearings of the compressor.
In general, a scroll compressor for use in a refrigerator or an air-conditioner has a motor-driven compressor unit composed of a scroll compressor section and a driving electric motor section which are coupled to each other through a rotary shaft supported by a frame. The motor-driven compressor unit as a whole is housed in a hermetic housing such that the compressor section overlies the motor section. The compressor has an orbiting scroll member having a disc-shaped end plate and a spiral wrap protruding uprightly therefrom and a stationary scroll member having a similar construction. The stationary scroll member has a discharge port and a suction port respectively formed in a central portion and a peripheral portion of the end plate thereof. Both scroll members are assembled together such that their wraps mesh with each other. A suction pipe is extended through the hermetic housing and connected to the suction port. An Oldham's mechanism is disposed between the orbiting scroll member and the frame so as to prevent the orbiting scroll member from rotating about its own axis. An eccentric shaft portion, provided on the upper end of the rotary shaft, engages with the orbiting scroll member such that the rotation of the rotary shaft causes an orbiting movement of the eccentric shaft portion which, in turn, causes an orbiting motion of the orbiting scroll member without allowing the orbiting member to rotate about its own axis. Consequently, the volume of closed compression chambers, formed by the wraps of both scroll members, is progressively decreased so that the gas confined in these chambers is progressively compressed and is discharged through the discharge port into a discharge chamber formed in the closed housing. The compressed gas is then introduced into the motor chamber which is formed in the lower portion of the housing. The lubricating oil suspended by the compressed gas is separated from the gas as the flow of the gas collides with, for example, the stationary part of the motor and the gas having no oil content is discharged to the outside of the compressor through a discharge pipe connected to the closed housing.
During the operation of this scroll compressor, the gas under compression produces a force which acts to urge both scroll members axially away from each other thus tending to form an axial gap between the axial ends and cooperating surfaces of the end plates of both scroll members. Consequently, the compressed gas in the compression chamber under higher pressure is allowed to escape into the compression chambers under lower pressure so that the compression performance of the compressor is impaired.
In order to overcome this problem, it has been proposed to apply a gas pressure to the back side of the orbiting scroll member such as to press the orbiting scroll member axially into close contact with the stationary scroll member. For instance, Japanese Patent Laid-Open No. 148994/1980 discloses an arrangement in which the intermediate pressure between the suction pressure and the discharge pressure is introduced into a back pressure chamber formed behind the end plate of the orbiting scroll member so as to produce the axial pressing force and thus attaining a tight seal between both scroll members.
The lubricating oil is collected in an oil pan formed in the bottom of the closed housing and is sucked therefrom by the pressure differential between the high pressure in the closed vessel and the intermediate pressure in the back-pressure chamber through an oil passage formed in the rotary shaft. The oil thus sucked is distributed to various parts requiring lubrication such as bearings.
A problem of this conventional scroll compressor, resides in the fact that the pressure at which the lubricating oil is supplied is reduced below a predetermined critical oil supply pressure in dependence on the operating condition of the compressor, particularly, when the compression ratio (ratio of discharge pressure to suction pressure) is low, for the reasons which will be described later.